The present invention relates to a process for reducing the salt concentration in a biomass-containing suspension, in a particular in a liquid containing cell fragments for obtaining proteins.
When obtaining intracellular proteins, disruption of the cells is followed by the use of aqueous phase systems composed of polyethylene glycol (PEG) and salt (phosphates, sulfates or citrates) in order to separate the cell fragments from the proteins and to concentrate the proteins. Aqueous solutions of polymer and salt (e.g., PEG with a molecular weight of 1,000-10,000 and potassium phosphate) separate above limiting concentrations into aqueous phases. Proteins, cells and cell fragments may be distributed differently in these phases depending on the concentration of the phase-forming substances. It is possible, by suitable choice of the phase system, to separate the cell fragments from the desired protein and to purify this protein further (M.-R. Kula et al., Adv. Biochem. Eng. 24:73-118 and German Patent 26 39 129).
This results in waste material in the form of a generally viscous suspension which contains cell fragments and in which between 10 and 25% (w/w) salt are dissolved, besides high molecular weight nucleic acids and soluble and insoluble protein. The salt should be recovered from this biomass-containing suspension in order both to reduce the costs of reagents and to minimize environmental pollution.
Since the cell fragments have a particle size between 0.05 and 5 .mu.m (lower limit not fixed), a small difference in density from the surrounding liquid, and cause a high viscosity, it is difficult to remove the salt mechanically. Available known processes are microfiltration, electrodialysis or heat-agglomeration of the lower phase, but implementation of these is not entirely satisfactory.
In the case of microfiltration, in view of the small particle size, it is necessary to use membranes with a very small effective separation size, in which case even with pressure differences of 1 bar across the membrane and flow-over rates of 5 m/s, the maximum flow rates achieved are only about 25 1/m.sup.2.h.
In the case of electrodialysis of such suspensions, the limiting current density decreases considerably as a consequence of formation of top layers composed of cell fragments on the diluate side of the membrane so that continuous operation is not technically achievable.
Even heat-agglomeration and separation of the agglomerate encounters considerable difficulties, especially separation problems, so that the use of this process is likewise unfeasible.